Access covers such as manhole covers are structures that are subjected to high loads during their use, and historically these structures have been made of steel or cast iron. Metal manhole covers are known to be heavy and awkward to handle. Often load bearing structures were made using various casting methods. The metal in a finished article produced by one of these methods has isotropic properties unless the metal has been subjected to some form of processing via physical treatment, such as for example, forging to create directional properties. However, in the vast majority of cases no processing is applied to the metal structure. Therefore, the isotropic properties of the metal being used are normally taken into consideration in the design of the load bearing structures.
FIG. 1 shows a typical reinforcing structure on the underside of a prior art metal manhole cover. It comprises a single beam 6 welded to the underside of a top plate 7. Four small beams 8 have also been welded to the top plate 7 with one end of each small beam 8 welded to one of the two longitudinal sides of the single beam 6. The single beam 6 carries most of the bending stress imposed by any load applied to the manhole cover. The four smaller beams 8 function as cross-beams and transfer the load from a first smaller beam 8 at one longitudinal side of the beam 6 to a second smaller beam 8 at the opposing longitudinal side. This is carried out via the single beam 6 which links the two opposing smaller beams 8.
In such a structure, if it is made for example from structural mild steel, e.g. hot rolled steel grade 4 (HR4), the material would have a yield tensile strength of 175 MN/M2. The bending stresses resulting from any load applied to the top plate 7 would produce in the single beam 6 on its outside face 6a tensile stresses in the direction of the length of the single beam 6. In the same manner the four smaller beams 8 would also be subject to tensile stresses in their longitudinal direction. The smaller beams 8 would also induce tensile stresses in the single beam 6 in the direction across the beam 6. Since the material is isotropic, tensile strength of the material on the outside surface 6a of the single beam 6 in the direction across the beam is the same as in its longitudinal direction, i.e. 175 MN/M2. In general, the access cover material is capable of accepting stresses determined by the design of the structure taking into account isotropic properties of the material.
Fibre composites have three inherent advantages compared to metal. They are light weight, possess low values of thermal conductivity and have high values of electrical resistance. In the access cover manufacturing industry there are well defined requirements for one or more of these characteristics.
Within the last twenty years fibre composites have been used successfully for the manufacture of manhole covers. Composite manhole covers are described in the specification of EP 0147050 B1. That invention provides a structure consisting of inner and outer skins separated by webs. The skins and the webs are made from fibre reinforced plastics material. The structure is closed at the edges and spaces between the webs are filled with a plastics foam material. The fibre structure within the webs has been further improved by the application of a three dimensional weaving technique disclosed in GB 2066308 (Cambridge Consultants Limited). The process of manufacturing such manhole covers is described in the specification of EP 0365579 B1.
Composite materials of the type made of long continuous fibre of typically glass or carbon and arranged into a specific construction within a matrix resin can be organized to have specific properties in specified directions. For example a beam made of glass fibre composite can have tensile properties of 500 MN/m2 in longitudinal direction and 150 MN/m2 in lateral direction. These values are influenced by the amount of fibre placed in any specific direction (the fibre architecture) as well as by the Fibre Volume Fraction (FVF) and the particular construction of the fibre. FVF indicates the quantity of fibre in a given volume, and the greater the amount of fibre, the higher the material performance achieved. High performance composites typically have an FVF from 50% to 60%. Fibres are arranged into a usable construction by weaving or stitch bonding. The construction influences the final performance of the composite material, although its influence is less than the influence from the fibre architecture or FVF. Because of their high tensile strength value, composite material have the advantage in that less material is required to manufacture a load bearing structure.
If composite materials were used to manufacture the manhole top of FIG. 1, the tensile strength along the length of the single beam 6 could reach 500 MN/m2. The same tensile strength could be achieved for the four small beams 8 in their longitudinal directions. As the single beam 6 is usually designed with a high value tensile strength in the longitudinal direction, the tensile strength in the direction across the single beam 6 i.e. perpendicular to the longitudinal direction would be reduced. At best the value that could be achieved would be typically 150 MN/m2. In use the two small beams 8 adjacent each longitudinal side of the single beam 6 transfer their loads to each other across the single beam 6. The single beam 6 has a significantly lower tensile strength value than the small beams 8 in the same direction. Therefore the problem associated with this kind of structure is that the regions of the single beam 6 between the ends of each pair of smaller beams 8 attached to the opposing longitudinal sides of the single beam 6 represent areas of weakness in the reinforcing structure. Additional structure/beams would be required to overcome this weakness.
Some manhole covers are used to cover access to steam, hot air or hot water pipelines. There are requirements for such manhole covers and similar load bearing structures to provide venting in the form of holes in their structure. This is impractical in the described above skin/web composite structure currently in extensive use. The alternative approach used in the prior art is to configure a conventional metal manhole cover in the form of a plate and provide a load bearing structure below it, where the venting can be provided in the areas of the plate that do not contain the load bearing structure.